The present invention relates to improvements in the coating of glass sheets or other substrates with thin films of a selected coating material and more particularly to the coating of such substrates by the cathode sputtering process.
The process of sputter-coating involves ion bombarding a target of the coating material in an ionized gas atmosphere in a chamber in which a controlled vacuum is maintained to cause atomic particles of the coating material to be dislodged and deposited on the substrates to be coated. One type of apparatus employed for carrying out the process embodies a plurality of aligned, independently controlled contiguous chambers including an entry chamber, a coating chamber and an exit chamber, sputter-coating means in the form of a substantially rectangular cathode mounted horizontally in the upper portion of the coating chamber and having a sheet or layer of a selected coating material carried by the bottom surface thereof and conveyor means for moving the substrates (glass sheets) in a horizontally disposed position through the successive chambers and beneath the sputter-coating means such that a continuous film of the selected coating material will be deposited on the upper surfaces of the substrates. The cathode comprises a housing including a top wall, a bottom wall and side and end walls which define a closed chamber therein. The walls of the cathode are preferably formed of aluminum of the desired thickness and are welded together along their meeting edges.
In such a process, the thin films deposited on the substrates are usually of a metal such as, for example, nickel, gold, chromium, copper, molybdenum or titanium, or alloys of these metals. Typical sputtering conditions are 5,000 volts potential on the target (coating material) and a resulting current density of 0.1 to 1.0 ma/cm.sup.2 for a time period of 0.2 to several minutes depending on the thickness of coating desired. It will be appreciated that this potential on the target in the ionized gas atmosphere of the coating chamber will generate a very high temperature which tends to heat the metal cathode to an undesirable degree. On the other hand, it is desirable that the cathode be operated as hot as possible without damage to the materials of which it is constructed since the heat aids in keeping the cathode in a clean and operable condition.
In order to maintain acceptable working conditions, it is essential that the cathodes be maintained at a temperature compatible with the materials of construction and to this end different methods have been proposed for effecting the cooling thereof. However, the cooling procedures heretofore utilized have not proven wholly satisfactory in meeting the requirements of continuous commercial operation.
A typical cooling procedure heretofore utilized has been to circulate ordinary tap water directly from the water main through the cathode chamber at a pressure of such magnitude that it will exert a pressure of about 3 lbs. p.s.i. on the walls of the cathode. However, this pressure together with the atmospheric pressure of about 14.7 lbs. p.s.i. on the water supply results in a total pressure of about 17.7 lbs. p.s.i. being imposed on the walls of the cathode. On the other hand, in the operation of a continuous sputter-coating apparatus of the character referred to above, the coating chamber is pumped down from atmosphere to a pressure usually in the range of 2 to 6.times.10.sup.- .sup.2 torr and preferably of about 4.times.10.sup.-.sup.2 torr, which pressure is maintained constant. The outward pressure exerted by the tap water on the walls of the cathode coupled with the vacuum created within the coating chamber places an undue stress on the walls of the cathode and more particularly on the welded joints securing the walls together. The resulting steady rate of stress, commonly referred to as creep, causes a gradual weakening and eventual rupture of the welded joints between the walls of the cathode resulting in leaks which necessitates removal of the cathode for repair and replacement. This is, of course, highly objectional not only from the standpoint of increased maintenance costs but also because of the loss of production time.
The present invention comprehends a new and improved procedure for internally cooling the cathodes employed in the sputter-coating of thin films of a material on glass sheets or other substrates.
Another object of the invention is the provision of an improved method and apparatus which involves the internal cooling of the cathode by circulating a cooling medium, such as oil, therethrough under a constant head pressure in such a manner as to reduce the pressure differential across the cathode walls and thereby minimize the stresses and resulting strains therein.
A further object of the invention is the provision of a method and apparatus in which air and other gases are continuously withdrawn from the cooling system as the oil circulates through the cathode such that the pressure of the oil upon the walls of the cathode is less than atmospheric pressure.